The art of heating, ventilating, and air conditioning (HVAC) system design has employed thermostats for many years in order to operate as a temperature sensitive switch to turn on or off and/or adjust other parameters on HVAC equipment. In the common manner in the art, a simple thermostat can be adjusted to establish a temperature set point such that, when the temperature in the conditioned space reaches the set point, the thermostat interacts with the HVAC equipment to take suitable action to heat or cool the conditioned space as may be appropriate.
Older generation, simple thermostats employed a temperature sensitive switch, such as a mercury switch, in order to make or break a circuit and, thereby, activate or deactivate HVAC equipment. Because of their relatively simple technology, these older generation thermostats did not require complicated building wiring and usually operated on a simple, two-wire system.
Modern HVAC systems offer numerous improvements to climate control in comparison to older systems. Modern thermostats, for example, offer many conveniences such as programmable temperature settings, timed activation/deactivation, and other such modern advances that were not available in older generation systems. Because of the increased complexity of modern HVAC systems, modern thermostats typically require four or more wires in order to operate. Two of these wires are commonly used to supply power to the thermostat while two or more other wires are commonly used for communications and/or data transfer with HVAC equipment.
A problem, thus, arises when retrofitting older buildings to install modern thermostats in buildings with older two-wire systems in place. New wiring, including more conductors, may be added to the systems in order to allow the newer equipment to operate. The conventional solution of removing the two-wire system and running new wiring to accommodate the newer thermostat, however, is undesirable as it is both expensive and time consuming. Thus, climate control manufacturers have sought means to operate modern thermostats using a two-wire system.
One such means is disclosed by U.S. Pat. No. 6,956,463 to Crenella et al. Crenella's patent discloses a method and apparatus that allows both power and communications to be conducted over a two-wire system between multiple low-voltage systems, such as thermostats. Crenella's method and apparatus operates, generally speaking, by using a two-wire system to supply AC power to the thermostat while replacing each half-cycle of the power transmission with a data transmission. In this manner, Crenella's device allows both power and communications to be supplied to a thermostat over a two-wire system. Crenella's system, however, does not support DC-powered thermostats because it relies upon the half-cycle of an AC power line to transmit the data.
Certain devices have also been directed to operating DC-powered thermostats over a two-wire system. U.S. Pat. No. 5,694,109 to Nguyen et al., for example, discloses a two-wire DC communications system that operates by using voltage variations in order to provide communications. Another such device is disclosed by U.S. Pat. No. 6,320,494 to Bartels et al., which discloses a two-way communications system over a pair of wires. A shunt placed within Bartels' circuit creates current variations that are detected by a microprocessor in the system. Bartels' system, however, creates variations in current through the shunt, and does not completely turn the current on and off.
Thus, there remains a long felt need in the art for a system that connects communications and DC-power to a modern thermostat through a conventional two-wired system by interrupting DC-current, rather than varying the amount of current.